Method and apparatus for transmitting signal via relay backhaul link

ABSTRACT

A method and a base station (BS) for transmitting a signal, and method and a relay for receiving a signal in a wireless communication system are discussed. The method for transmitting a signal according to an embodiment includes mapping a reference signal to a subframe comprising a first slot and a second slot. Each of the first and second slots includes a plurality of resource elements over which the reference signal is allowed to be mapped. When a last orthogonal frequency division multiplexing (OFDM) symbol of the subframe is not available for a BS-to-relay transmission, the reference signal is not mapped to the second slot and a data signal is mapped to the resource elements of a penultimate OFDM symbol of the second slot. The method further includes transmitting the reference signal to a relay.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.14/532,788 filed on Nov. 4, 2014 (now U.S. Pat. No. 9,510,351 issued onNov. 29, 2016), which is a Continuation of U.S. patent application Ser.No. 13/518,539 filed on Jun. 22, 2012 (now U.S. Pat. No. 8,908,590issued on Dec. 9, 2014), which is filed as the National Phase ofPCT/KR2011/000610 filed on Jan. 28, 2011, which claims the benefit under35U.S.C. §119(e) to U.S. Provisional Application No. 61/299,323 filed onJan. 28, 2010, and under 35 U.S.C. §119(a) to Korean Patent ApplicationNo. 10-2011-0008625 filed on Jan. 28, 2011, all of which are herebyexpressly incorporated by reference into the present application.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a method and apparatus for transmitting a signal via arelay backhaul link.

Discussion of the Related Art

Wireless communication systems have been widely deployed to providevarious types of communication services including voice and dataservices. In general, a wireless communication system is a multipleaccess system that supports communication of multiple users by sharingavailable system resources (e.g. a bandwidth, transmission power, etc.)among the multiple users. The multiple access system may adopt amultiple access scheme such as Code Division Multiple Access (CDMA),Frequency Division Multiple Access (FDMA), Time Division Multiple Access(TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), or SingleCarrier Frequency Division Multiple Access (SC-FDMA).

SUMMARY OF THE INVENTION

An object of the present invention devised to solve the problem lies ona method and apparatus for efficiently transmitting a signal in a relaysystem.

Another object of the present invention devised to solve the problemlies on a method and apparatus for efficiently transmitting a referencesignal and/or data in a relay system.

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present invention could achieve will be more clearlyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings.

The object of the present invention can be achieved by providing amethod for transmitting a signal to a relay at a Base Station (BS) in awireless communication system, including mapping a Reference Signal (RS)to a subframe having two slots, and transmitting the subframe to therelay. Each of the slots includes a plurality of consecutive resourceelements over which the RS can be spread and the plurality ofconsecutive resource elements are overlapped with a last OrthogonalFrequency Division Multiplexing (OFDM) symbol of the slot. If a lastOFDM symbol of the subframe is not available to the relay, the RS istransmitted only in a first slot of the subframe.

In another aspect of the present invention, provided herein is a BS in awireless communication system, including a Radio Frequency (RF) unit anda processor. The processor is adapted to map an RS to a subframe havingtwo slots, and transmit the subframe to the relay. Each of the slotsincludes a plurality of consecutive resource elements over which the RScan be spread and the plurality of consecutive resource elements areoverlapped with a last OFDM symbol of the slot. If a last OFDM symbol ofthe subframe is not available to the relay, the RS is transmitted onlyin a first slot of the subframe.

If the last OFDM symbol of the subframe is available to the relay, theRS may be transmitted in the two slots of the subframe.

The plurality of consecutive resource elements may be consecutive intime in each of the slots.

The plurality of consecutive resource elements may be two resourceelements consecutive in time in each of the slots.

If the RS is transmitted only in the first slot of the subframe, a datasignal may be mapped to at least part of the plurality of consecutiveresource elements in which the reference signal can be spread in asecond slot of the subframe. In this case, the data signal may be mappedto remaining resource elements except resource elements overlapped withthe last OFDM symbol of the subframe among the plurality of consecutivereference elements in the second slot of the subframe. The data signalmay be spread with an orthogonal code used for transmitting the RS inthe plurality of consecutive resource elements in the second slot of thesubframe.

In accordance with embodiments of the present invention, a signal can beefficiently transmitted in a relay system. Particularly, a referencesignal and/or data can be efficiently transmitted in the relay system.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 illustrates a network configuration for an Evolved UniversalMobile Telecommunications System (E-UMTS) system.

FIG. 2 illustrates a radio frame structure in the E-UMTS system.

FIG. 3 illustrates the structure of a resource grid for a radio frame.

FIG. 4 illustrates a downlink subframe structure.

FIG. 5 illustrates a signal transmission operation in a Multiple InputMultiple Output (MIMO) scheme.

FIG. 6 illustrates downlink Reference Signal (RS) patterns in a LongTerm Evolution (LTE) system.

FIG. 7 illustrates a Demodulation Reference Signal (DRS) structure addedto an LTE-Advanced (LTE-A) system.

FIG. 8 illustrates a wireless communication system having relays.

FIG. 9 illustrates an exemplary backhaul transmission in a MulticastBroadcast Single Frequency Network (MBSFN) subframe.

FIG. 10 illustrates an exemplary problem that is produced duringDeModulation Reference Signal (DM RS) reception, when a relay fails toreceive the last Orthogonal Frequency Division Multiplexing (OFM) symbolof a backhaul subframe.

FIGS. 11 and 12 are flowcharts illustrating DM RS transmissionoperations of an evolved Node B (eNB) according to an embodiment of thepresent invention.

FIG. 13 is a flowchart illustrating a channel estimation operation of arelay according to an embodiment of the present invention.

FIGS. 14 and 15 illustrate exemplary signal transmissions in the secondslot of a subframe, when the last OFDM symbol of the subframe is notavailable to a relay.

FIG. 16 is a flowchart illustrating a DM RS transmission operation of aneNB according to another embodiment of the present invention.

FIG. 17 is a flowchart illustrating a channel estimation operation of aUser Equipment (UE) according to another embodiment of the presentinvention.

FIG. 18 illustrates an exemplary signal transmission in the second slotof a subframe, when DM RS Transmission (Tx) is disabled for the secondslot of the subframe.

FIG. 19 is a block diagram of a Base Station (BS), a relay or Relay Node(RN), and a UE that are applicable to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Embodiments of the present invention are applicable to avariety of wireless access technologies such as Code Division MultipleAccess (CDMA), Frequency Division Multiple Access (FDMA), Time DivisionMultiple Access (TDMA), Orthogonal Frequency Division Multiple Access(OFDMA), and Single Carrier Frequency Division Multiple Access(SC-FDMA). CDMA can be implemented as a radio technology such asUniversal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA can beimplemented as a radio technology such as Global System for Mobilecommunications (GSM)/General Packet Radio Service (GPRS)/Enhanced DataRates for GSM Evolution (EDGE). OFDMA can be implemented as a radiotechnology such as Institute of Electrical and Electronics Engineers(IEEE) 802.11 (Wireless Fidelity (Wi-Fi)), IEEE 802.16 (Worldwideinteroperability for Microwave Access (WiMAX)), IEEE 802.20, EvolvedUTRA (E-UTRA). UTRA is a part of Universal Mobile TelecommunicationsSystem (UMTS). 3rd Generation Partnership Project (3GPP) Long TermEvolution (LTE) is a part of Evolved UMTS (E-UMTS) using E-UTRA,employing OFDMA for downlink and SC-FDMA for uplink. LTE-Advanced(LTE-A) is an evolution of 3GPP LTE.

While the following description is given, centering on 3GPP LTE/LTE-A toclarify the description, this is purely exemplary and thus should not beconstrued as limiting the present invention.

FIG. 1 illustrates a network configuration for an E-UMTS system. E-UMTSis an evolution of Wideband CDMA (WCDMA) UMTS and the 3GPP is working onstandardization of E-UMTS. E-UMTS is also called LTE. For details ofUMTS and E-UMTS technical specifications, refer respectively to Release7 and Release 8 of “3rd Generation Partnership Project; TechnicalSpecification Group Radio Access Network”.

Referring to FIG. 1, the E-UMTS system includes a User Equipment (UE)120, evolved Node Bs (eNBs or eNode Bs) 110 a and 110 b, and an AccessGateway (AG) which is connected to an external network, at an end of anEvolved UMTS Terrestrial Radio Access Network (E-UTRAN). An eNB cansimultaneously transmit multiple data streams for a multicast serviceand/or a unicast service. One eNB manages one or more cells (e.g. threecells). A cell is configured to provide a downlink or uplinktransmission service to a plurality of UEs in one of bandwidths 1.4, 3,5, 10, 15 and 20 MHz. Different cells may be set to differentbandwidths. An eNB controls data transmission and reception for aplurality of UEs. For downlink data, the eNB notifies a UE of atime/frequency area to carry the downlink data, a coding scheme, a datasize, Hybrid Automatic Repeat reQuest (HARQ)-related information, etc.by transmitting downlink scheduling information. For uplink data, theeNB notifies a UE of a time/frequency area available to the UE, a codingscheme, a data size, HARQ-related information, etc. by transmittinguplink scheduling information. An interface may be established betweeneNBs to transmit user traffic or control traffic. A Core Network (CN)may include an AG and a network node for user registration of UEs. TheAG manages the mobility of a UE on a Tracking Area (TA) basis. A TAincludes a plurality of cells.

FIG. 2 illustrates a radio frame structure in the E-UMTS system.

Referring to FIG. 2, the E-UMTS system uses a 10-ms radio frame. Theradio frame is divided into 10 subframes. Each subframe is furtherdivided into two slots, each being 0.5 ms in duration and having aplurality of symbols (e.g. OFDM symbols or SC-FDMA symbols).

FIG. 3 illustrates the structure of a resource grid for the duration ofone slot.

Referring to FIG. 3, a slot includes a plurality of OFDM symbols orSC-FDMA symbols in time by a plurality of Resource Blocks (RBs) infrequency. One RB has 12×7(6) Resource Elements (REs). The number of RBsin a time slot depends on a bandwidth set for a cell. Each element inthe resource grid is referred to as an RE. An RE is the smallest unit ofresources, including one subcarrier for a duration of one symbol. Whilea time slot and an RB are shown in FIG. 3 as including 7 symbols and 12subcarriers, respectively, this is purely exemplary and thus does notlimit the present invention. For example, the number of symbols per slotdepends on the length of a Cyclic Prefix (CP).

FIG. 4 illustrates a downlink subframe structure.

Referring to FIG. 4, a Layer 1 (L1)/Layer 2 (L2) control region ismultiplexed with a data region in Time Division Multiplexing (TDM) in adownlink subframe in an LTE system. The L1/L2 control region occupiesthe first n OFDM symbols (e.g. the first three or four OFDM symbols) ofthe downlink subframe and the data region occupies the remaining OFDMsymbols of the downlink subframe. The L1/L2 control region includes aPhysical Downlink Control CHannel (PDCCH) for carrying downlink controlinformation and the data region includes a downlink data channel,Physical Downlink Shared CHannel (PDSCH). To receive a downlink signal,a UE reads downlink scheduling information from the PDCCH. Then the UEreceives downlink data on the PDSCH based on resource allocationinformation indicated by the downlink scheduling information. Resourcesscheduled for the UE (i.e. the PDSCH) are allocated on an RB basis or onan RB group basis.

The PDCCH delivers information related to resource allocation fortransport channels, a Paging CHannel (PCH) and a Downlink Shared CHannel(DL-SCH), an uplink scheduling grant, and HARQ information to the UE.Control information carried on the PDCCH is generically called DownlinkControl Information (DCI). Various DCI formats are defined according tothe contents of DCI.

Table 1 illustrates DCI format 0 for uplink scheduling.

TABLE 1 Field Bits Comment Format 1 Uplink grant or downlink assignmentHopping flag 1 Frequency hopping on/off RB assignment 7 Resource blockassigned for PUSCH MCS 5 Modulation scheme, coding scheme, etc. New DataIndicator 1 Toggled for each new transport block TPC 2 Power control ofPUSCH Cyclic shift for DMRS 3 Cyclic shift of demodulation referencesignal CQI request 1 To request CQI feedback through PUSCH RNTI/CRC 1616 bit RNTI implicitly encoded in CRC Padding 1 To ensure format 0matches format 1A in size Total 38 — MCS: Modulation and Coding Scheme.TPC: Transmit Power Control RNTI: Radio Network Temporary IdentifierCRC: Cyclic Redundancy Check

A UE for which a PDCCH is destined is identified by an RNTI. Forinstance, on the assumption that the CRC of a PDCCH is masked by RNTI Aand the PDCCH delivers uplink resource allocation information B (e.g.frequency positions) and transport format information C (e.g. atransport block size, a modulation scheme, coding information, etc.),UEs monitor PDCCHs using their own RNTIs within a cell and a UE havingRNTI A transmits an uplink signal based on the information B and Cacquired from the PDCCH with RNTI A.

FIG. 5 illustrates an exemplary signal transmission operation accordingto a Multiple Input Multiple Output (MIMO) scheme.

Referring to FIG. 5, scramblers 301 scramble codewords. Each codewordincludes a coded bit stream corresponding to a transport block.Modulation mappers 302 modulate the scrambled codewords to complexsymbols in Binary Phase Shift Keying (BPSK), Quadrature Phase ShiftKeying (QPSK), 16-ary Quadrature Amplitude Modulation (16 QAM), or64-ary Quadrature Amplitude Modulation (64 QAM) according to the type ofthe transmission signal and/or a channel state. A layer mapper 303 mapsthe complex symbols to one or more layers.

In the case of signal transmission through a single antenna, onecodeword is mapped to one layer. In the case of signal transmissionthrough multiple antennas, the codeword-to-layer mapping relationshipmay vary depending on a transmission scheme. Table 2 and Table 3illustrate exemplary codeword-to-layer mapping relationships.

TABLE 2 Number Number of of code Codeword-to-layer mapping layers wordsi = 0, 1, . . . , M_(symb) ^(layer) − 1 1 1 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb)^(layer) = M_(symb) ⁽⁰⁾ 2 2 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb) ^(layer) =M_(symb) ^((0) = M) _(symb) ⁽¹⁾ x⁽¹⁾(i) = d⁽¹⁾(i) 2 1 x⁽⁰⁾(i) = d⁽⁰⁾(2i)M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) 3 2 x⁽⁰⁾(i) =d⁽⁰⁾(i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾ = M_(symb) ⁽¹⁾/2 x⁽¹⁾(i) =d⁽¹⁾(2i) x⁽²⁾(i) = d⁽¹⁾(2i + 1) 4 2 x⁽⁰⁾(i) = d⁽⁰⁾(2i) M_(symb) ^(layer)= M_(symb) ⁽⁰⁾/2 = M_(symb) ⁽¹⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) x⁽²⁾(i) =d⁽¹⁾(2i) x⁽³⁾(i) = d⁽¹⁾(2i + 1)

TABLE 3 Number of Number code Codeword-to-layer mapping of layers wordsi = 0,1, . . . , M_(symb) ^(layer) − 1 2 1 x⁽⁰⁾(i) = d⁽⁰⁾(2i) M_(symb)^(layer) = M_(symb) ⁽⁰⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) 4 1 x⁽⁰⁾(i) = d⁽⁰⁾(4i)x⁽¹⁾(i) = d⁽⁰⁾(4i + 1) $M_{symb}^{layer} = \{ \begin{matrix}{M_{symb}^{(0)}/4} & {{{if}\mspace{14mu} M_{symb}^{(0)}\;{mod}\mspace{11mu} 4} = 0} \\{( {M_{symb}^{(0)} + 2} )/4} & {{{if}\mspace{14mu} M_{symb}^{(0)}\;{mod}{\;\;}4} \neq 0}\end{matrix} $ x⁽²⁾(i) = d⁽⁰⁾(4i + 2) If M_(symb) ⁽⁰⁾ mod 4 ≠ 0two null symbols shall be x⁽³⁾(i) = d⁽⁰⁾(4i + 3) appended tod⁽⁰⁾(M_(symb) ⁽⁰⁾ − 1)

Table 2 describes codeword-to-layer mapping for spatial multiplexing andTable 3 describes codeword-to-layer mapping for transmit diversity. InTable 2 and Table 3, x(a)(i) represents an i^(th) symbol of a layer withindex a, and d(a)(i) represents an i^(th) symbol of a codeword withindex a.

As noted from Table 2 and Table 3, one codeword may be mapped to onelayer on a symbol basis. However, as in the second case of Table 3, onecodeword may be distributed to up to four layers. In the distributedcodeword-to-layer mapping, the symbols of each codeword are sequentiallymapped to layers.

While Table 2 and Table 3 are based on the assumption of up to twocodewords and up to four layers, this is illustrative. Thus the maximumnumbers of codewords and layers for signal transmission may varydepending on systems.

A precoder 304 multiplies the layer-mapped signals by a precoding matrixselected according to a channel state and allocates the multipliedsignals to transmission antennas. RE mappers 305 map the transmissionsignals for the respective antennas to time-frequency REs. Then OFDMsignal generators 306 transmit the mapped transmission signals throughthe respective antennas.

FIG. 6 illustrates downlink Reference Signal (RS) patterns in the LTEsystem.

Referring to FIG. 6, two types of downlink RSs are defined for a unicastservice in the LTE system, Common RSs (CRSs) 0 to 3 targeting channelstate information acquisition and measurements, for example, forhandover and UE-specific RSs targeting data modulation. The UE-specificRSs are also called dedicated RSs (DRSs). The UE-specific RSs are usedto demodulate beamforming data. The CRSs are used for both channelinformation acquisition and data demodulation. The CRSs arecell-specific and transmitted over a total frequency band in everysubframe. Because the LTE system supports up to four Transmission (Tx)antennas on downlink, CRSs for up to four antenna ports may betransmitted according to the number of Tx antennas at an eNB. CRSs aretransmitted through antenna ports 0 to 3 and a UE-specific RS D istransmitted through antenna port 5 in the LTE system.

The LTE-A system evolved from the LTE system should be able to supportup to eight Tx antennas on downlink. Therefore, the LTE-A system shouldsupport RSs for up to eight Tx antennas. Since downlink RSs are definedonly for up to four Tx antennas in the LTE system, RSs should beadditionally defined for additional antenna ports, when an eNB has fourto eight downlink Tx antennas in the LTE-A system.

FIG. 7 illustrates an exemplary pattern of Demodulation ReferenceSignals (DM RSs) added to the LTE-A system. A DM RS is a UE-specific RSused to demodulate each layer signal, when signals are transmittedthrough multiple antennas. DM RSs are used for demodulation of a PDSCHand a Relay-PDSCH (R-PDSCH). Since the LTE-A system uses up to eight Txantennas, it needs up to eight layers and DM RSs for the respectivelayers. For the sake of convenience, DM RSs for layers 0 to 7 arereferred to as DM RSs (layers) 0 to 7.

Referring to FIG. 7, DM RSs for two or more layers are multiplexed inCode Division Multiplexing (CDM) over the same REs. To be more specific,the DM RSs for the respective layers are spread with spreading codes(e.g. orthogonal codes such as Wash codes or Discrete Fourier Transform(DFT) codes) and then multiplexed in the same REs. For instance, a DM RSfor layer 0 and a DM RS for layer 1 are multiplexed in the same REs.Specifically, the DM RSs for layer 0 and layer 1 are spread withorthogonal codes at subcarrier 1 (k=1) in two OFDM symbols 12 and 13.That is, the DM RSs for layer 0 and layer 1 are spread with codes withSpreading Factor (SF)=2 in time and multiplexed in the same REs in eachslot. The spreading codes for the DM RSs for layer 0 and layer 1 may be,for example, [+1+1] and [+1−1], respectively. Similarly, DM RSs forlayer 2 and layer 3 are spread with different orthogonal codes in thesame REs. DM RSs for layers 4, 5, 6 and 7 are spread with codesorthogonal to the spreading codes of the DM RSs for layers 0, 1, 2 and 3in the REs occupied by the DM RSs 0 & 1 and the DM RSs 2 & 3. For up tofour layers, codes with SF=2 are used for DM RSs, whereas for five ormore layers, codes with SF=4 are used for DM RSs. In the LTE-A system,antenna ports for DM RSs are given as {7, 8, . . . , n+6} (n is thenumber of layers).

Table 4 below lists spreading sequences for antenna ports 7 to 14defined in LTE-A.

TABLE 4 Antenna port p [w _(p) (0) w _(p) (1) w _(p) (2) w _(p) (3)] 7[+1 +1 +1 +1] 8 [+1 −1 +1 −1] 9 [+1 +1 +1 +1] 10 [+1 −1 +1 −1] 11 [+1 +1−1 −1] 12 [−1 −1 +1 +1] 13 [+1 −1 −1 +1] 14 [−1 +1 +1 −1]

Referring to Table 4, the same orthogonal code with length 2 is repeatedin each of orthogonal codes for antenna ports 7 to 10. As a consequence,orthogonal codes with length 2 are used at a slot level for up to fourlayers. For five or more layers, orthogonal codes with length 4 are usedat a subframe level.

FIG. 8 illustrates a wireless communication system having relays. Arelay or Relay Node (RN) extends the service area of an eNB or isinstalled in a shadowing area to thereby provide a reliable service.

Referring to FIG. 8, the wireless communication system includes an eNB,relays, and UEs. The UEs communicate with the eNB or the relays. For thesake of convenience, a UE communicating with an eNB is referred to as amacro UE and a UE communicating with a relay is referred to as a relayUE. A communication link between an eNB and a macro UE and acommunication link between a relay and a relay UE are referred to as amacro access link and a relay access link, respectively. A communicationlink between an eNB and a relay is referred to as a backhaul link.

FIG. 9 illustrates an exemplary backhaul transmission in a MulticastBroadcast Single Frequency Network (MBSFN) subframe. For in-bandrelaying, an eNB-to-relay link (i.e. a backhaul link) operates in thesame frequency band as a relay-to-UE link (i.e. a relay access link). Inthe case where a relay transmits a signal to a UE while it is receivinga signal from an eNB or vice versa, the transmitter and receiver of therelay interfere mutually. Accordingly, simultaneous eNB-to-relay andrelay-to-UE transmissions on the same frequency resources may belimited. For this purpose, the backhaul link and the relay access linkare partitioned in Time Division Multiplexing (TDM). In the LTE-Asystem, a backhaul link is established in an MBSFN subframe to supportmeasurements of legacy LTE UEs located in a relay zone (fake MBSFN). Ifa subframe is signaled as an MBSFN subframe, a UE receives only thecontrol region (ctrl) of the subframe and thus the relay may configure abackhaul link using the data region of the subframe.

Embodiment 1

Due to propagation delay between an eNB and a relay,Reception/Transmission (Rx/Tx) switching of the relay, system setting,the relay may not receive the last OFDM symbol of a backhaul subframe.This is because the relay should switch from an Rx mode to a Tx mode atthe time of the last OFDM symbol in order to transmit the first OFDMsymbol of the next subframe.

FIG. 10 illustrates a problem produced during DM RS reception, when arelay fails to receive the last OFDM symbol of a backhaul subframe.

Referring to FIG. 10, if the relay fails to receive the last OFDM symbolwith index 13 of a backhaul subframe, the relay does not receive a partof REs allocated to a DM RS. As described before with reference to FIG.7, DM RSs for two or more layers share the same REs in CDM. Therefore,if the relay does not receive OFDM symbol 13, REs carrying DM RSs inOFDM symbol 12 are not helpful in channel estimation. Without REs ofOFDM symbol 13, the relay cannot separate a plurality of DM RSsmultiplexed in REs of OFDM symbols 12 and 13 through despreading. As aresult, the DM RS REs of OFDM symbol 12 causes unnecessary overhead forbackhaul transmission and reception, thereby wasting resources.

To avert this problem, the relay may use DM RS REs of the second lastOFDM symbol of a subframe as data REs, if the relay cannot receive thelast OFDM symbol of the subframe. For example, if the relay is notcapable of receiving OFDM symbol 13 as illustrated in FIG. 10, the eNBtransmits a data signal in the DM RS REs of OFDM symbol 12 and the relaydecodes its own data including the data carried in the DM RS REs of OFDMsymbol 12. In this case, the relay performs channel estimation usingonly DM RS REs in the first slot of the subframe and decodes the data ofthe first and second slots based on the channel estimation. That is, ifthe relay cannot receive the last OFDM symbol of a subframe, DM RSs aretransmitted only in the first slot of the subframe. Consequently,utilization of radio resources can be increased.

FIGS. 11, 12 and 13 are flowcharts illustrating signal processingoperations according to an embodiment of the present invention.Specifically, FIGS. 11 and 12 illustrate DM RS transmission operationsat an eNB and FIG. 13 illustrates a channel estimation operation at arelay.

Referring to FIG. 11, the eNB generates a DM RS sequence (or DM RSsequences) for each layer (S1110). The DM RS sequence may be, but notlimited to, a pseudo-random sequence, a Zadoff-chu sequence, or aConstant Amplitude Zero Auto Correlation (CAZAC) sequence. For example,referring to generation of an RS sequence for antenna port 5 in thelegacy LTE system, the DM RS sequence may be defined as

$\begin{matrix}{{r(m)} = {{\frac{1}{\sqrt{2}}( {1 - {2 \cdot {c( {2m} )}}} )} + {j\frac{1}{\sqrt{2}}( {1 - {2 \cdot {c( {{2m} + 1} )}}} )}}} & \lbrack {{Math}\mspace{14mu}{Figure}\mspace{14mu} 1} \rbrack\end{matrix}$

where m is 0 or a larger integer and c(m) is a pseudo-random sequencegiven by [Equation 2]. The pseudo-random sequence is defined by Goldsequences of length 31.c(n)=(x ₁(n+N _(C))+x ₂(n+N _(C))mod2x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₂(n+1)+x ₂(n))mod 2  [Math Figure 2]

where Nc=1600 and n=1, 2, . . . , 30. The first Gold sequence may beinitialized to x₁(0)=1, x₁(n)=0 and the second Gold sequence may beinitialized to c_(init)=(└n_(s)/2┘+1)·(2N_(ID) ^(cell)+1)·2¹⁶+α. n_(s)denotes a slot index, N_(ID) ^(cell) denotes a cell ID, and a is aconstant.

Then the eNB determines whether the relay is capable of using the lastsymbol of a subframe (S1120). The determination may be made in adifferent manner according to how subframe boundaries of the eNB and therelay are configured. Therefore, whether or not the last symbol isavailable to the relay may be indicated by system information or RadioResource Control (RRC) signaling. If the last symbol of the subframe isavailable to the relay, the eNB transmits DM RSs in the first and secondslots of the subframe (S1130). In this case, the DM RSs may betransmitted in the manner illustrated in FIG. 7. On the other hand, ifthe last symbol of the subframe is not available to the relay, the eNBtransmits a DM RS (or DM RSs) only in the first slot of the subframe(S1140). In other words, the BS does not transmit a DM RS (or DM RSs) inthe second slot of the subframe to the relay. In this case, data (anR-PDSCH) may be mapped to DM RS REs of the second slot intended for therelay. Transmission processing of the DM RSs may include, but is notlimited to, precoding, RE mapping, and OFDM signal generation.

Referring to FIG. 12, the BS may operate differently according to signalrecipients. For the sake of convenience, it is assumed that a relaycannot use the last OFDM symbol of a subframe. The eNB basicallyoperates in a similar manner to that illustrated in FIG. 11. First, theeNB generates a DM RS sequence (or DM RS sequences) for each layer(S1210). Then the eNB determines a receiving end to receive a DM RS (orDM RSs) (S1220). If the eNB is to transmit the DM RS to a macro UE, theBS transmits the DM RS in the first and second slots of the subframe,for example, in the manner illustrated in FIG. 7 (S1230). On the otherhand, if the eNB is to transmit the DM RS to a relay, the eNB transmitsthe DM RS only in the first slot of the subframe to the relay (S1240).That is, no DM RSs are transmitted in the second slot of the subframe tothe relay. In this case, data (an R-PDSCH) may be mapped to DM RS REs ofthe second slot of the subframe, intended for the relay. While theprocedures of FIGS. 11 and 12 have been described separately, they maybe combined into one procedure.

Referring to FIG. 13, the relay receives a subframe including a DM RS(or DM RSs) from the eNB (S1310). The subframe is a backhaul subframe,preferably an MBSFN subframe. The relay determines whether it can usethe last symbol of the subframe (S1320). Whether or not the relay canuse the last symbol of the subframe is predetermined or indicated bysystem information or RRC signaling. If the relay can use the lastsymbol of the subframe, the relay performs channel estimation using a DMRS (or DM RSs) received in the first and second slots of the subframe(S1330). In this case, the DM RS (or DM RSs) may be received in themanner illustrated in FIG. 7. On the other hand, if the relay cannot usethe last symbol of the subframe, the relay performs channel estimationbased on a DM RS (or DM RSs) received in the first slot of the subframe(S1340). That is, no DM RSs are received in the second slot of thesubframe. In this case, data (an R-PDSCH) may be mapped to DM RS REs ofthe second slot of the subframe, intended for the relay.

FIGS. 14 and 15 illustrate exemplary signal transmissions in the secondslot of a subframe, when the last OFDM symbol of the subframe is notavailable to a relay. When the afore-described operation is performed, adirect link signal for a UE connected directly to an eNB and a backhaulsignal for a relay may be simultaneously transmitted through differentlayers in the same RB (MultiUser MIMO (MU-MIMO)). In this case, anadditional operation is needed to help the UE with accurate DM RSdespreading. For this purpose, a backhaul data signal in OFDM symbol 12is spread with a CDM code used for a DM RS corresponding to the layer ofthe backhaul data signal and transmitted in a DM RS RE of OFDM symbol13. This means that the eNB spreads a data signal for a backhaul linkwith a CDM code used for a DM RS corresponding to the layer of the datasignal in OFDM symbols 12 and 13.

Referring to FIGS. 14 and 15, it is assumed that a backhaul signal useslayer 0 and a direct link signal uses layer 1 in MU-MIMO. It is alsoassumed that the signals of layers 0 and 1 are spread with CDM codes[w_(0,0) w_(0,1)] and [w_(1,0) w_(1, 1)], respectively in twoconsecutive DM RS REs. If a data signal S1,12 is to be transmitted as abackhaul signal at subcarrier 1 (k=1) in OFDM symbol 12, the eNBtransmits a signal w_(0,0)*S_(1,12) at subcarrier 1 in OFDM symbol 12and a spread version of the data signal S1,12, w0,1*S1,12 at subcarrier1 in OFDM symbol 13. To facilitate the relay to detect the data signal,the backhaul link DM RS spreading code for OFDM symbol 12 and OFDMsymbol 13 is subjected to appropriate phase rotation such that thesymbol phase of the spreading code is 0 in OFDM symbol 12 (that is, aCDM code [1 w_(0,1)/w_(0,0)] is used to thereby multiply S1,12 by 1 inthe above example).

From the perspective of the relay, the relay simply discards the lastOFDM symbol of a subframe and demodulates/decodes an R-PDSCH,considering that a data signal is also carried in DM RS REs of thesecond last OFDM symbol of the subframe. Meanwhile, from the perspectiveof a UE connected directly to the eNB, if a backhaul signalMU-MIMO-operated with a signal for the UE is transmitted in DM RS REs,the backhaul signal is spread with a code orthogonal to a DM RS of theUE, irrespective of whether the backhaul signal is a data signal or anRS. Therefore, the UE despreads a signal in its DM RS RE and performschannel estimation using the despread signal, as is done when its signalis MU-MIMO-operated with a signal for another UE.

The above-described operation is also applicable to a subframe with 12OFDM symbols in the case of an extended CP.

Embodiment 2

When a UE is connected directly to an eNB, that is, an access link isestablished between the UE and the eNB, the eNB transmits DM RSs in bothslots of a subframe, as illustrated in FIG. 7. However, DM RSs may notneed to be transmitted in both slots of the subframe undercircumstances. For example, if a channel changes slowly or is static, noproblems may occur to data demodulation even though a channel in anotherslot is estimated using a DM RS in one slot. Accordingly, DM RS Tx in aslot is selectively enabled or disabled in this embodiment. Thus, DM RSoverhead can be reduced.

FIGS. 16 and 17 illustrate signal processing operations according toanother embodiment of the present invention. Specifically, FIG. 16illustrates a DM RS transmission operation of an eNB and FIG. 17illustrates a channel estimation operation of a UE.

Referring to FIG. 16, the eNB generates a DM RS sequence (or DM RSsequences) for each layer (S1610). The DM RS sequence may be, but is notlimited to, a pseudo-random sequence, a Zadoff-chu sequence, or a CAZACsequence. Referring to RS sequence generation for antenna port 5 in thelegacy LTE system, for example, the DM RS sequence may be defined using[Equation 1] and [Equation 2].

Then the BS determines whether DM RS Tx is disabled in the second slotof a subframe for the UE (S1620). DM RS Tx disable/enable may be set bya higher layer (e.g. an RRC layer) or a physical layer. DM RS Txdisable/enable may be signaled to a UE in various manners. For instance,the DM RS Tx disable/enable may be indicated semi-statically to the UEthrough higher layer signaling (e.g. RRC signaling). Additionally, theDM RS Tx disable/enable may be indicated dynamically to the UE throughphysical layer signaling (e.g. via a PDCCH for DL allocation).Furthermore, information indicating that DM RS disable is allowed andinformation about the start and duration of the DM RS disable may betransmitted by higher layer signaling and actual DM RS Tx disable/enablemay be indicated by physical layer signaling. The DM RS Txdisable/enable may be set, taking into account a channel state (e.g.whether a channel state is (semi-)static).

If the DM RS Tx is not disabled, that is, enabled for the UE in thesecond slot of a subframe, the eNB transmits a DM RS (or DM RSs) to theUE in the first and second slots of the subframe, for example, in themanner illustrated in FIG. 7 (S1630). On the other hand, if the DM RS Txis disabled in the second slot for the UE, the eNB transmits a DM RS (orDM RSs) only in the first slot of the subframe (S1640). That is, no DMRSs are transmitted in the second slot of the subframe. In this case,data (a PDSCH) may be mapped to the positions of DM RSs in the secondslot of the subframe. Transmission processing of the DM RSs may include,but is not limited to, precoding, RE mapping, and OFDM signalgeneration.

Referring to FIG. 17, the UE receives a subframe including a DM RS (orDM RSs) from the eNB (S1710). The UE determines whether DM RS Tx isdisabled for the second slot of the subframe (S1720). DM RS Txdisable/enable may be set through various types of signaling describedwith reference to FIG. 16. If the DM RS Tx is enabled for the secondslot of the subframe, the UE performs channel estimation using a DM RS(or DM RSs) received in the first and second slots of the subframe(S1730). In this case, the DM RS (or DM RSs) may be received in themanner illustrated in FIG. 7. On the other hand, if the DM RS Tx isdisabled for the second slot of the subframe, the UE performs channelestimation based on a DM RS (or DM RSs) received in the first slot ofthe subframe (S1740). That is, no DM RSs are received in the second slotof the subframe. In this case, data (an PDSCH) may be mapped to thepositions of DM RSs in the second slot of the subframe.

FIG. 18 illustrates an exemplary signal transmission in the second slotof a subframe, when DM RS Tx is disabled for the second slot of thesubframe. For the sake of convenience, it is assumed that macro UE Auses layer 0 and macro UE B uses layer 1 in MU-MIMO. It is also assumedthat signals of layers 0 and 1 are spread with CDM codes [w_(0,0)w_(0,1)] and [w_(1,0) w_(1,1)], respectively in two consecutive DM RSREs. It is also assumed that DM RS Tx in the second slot of a subframeis disabled for UE A and enabled for UE B.

Referring to FIG. 18, if a data signal Sk,12 is to be transmitted to UEA at subcarrier k (k=1, 6, 11) in OFDM symbol 12, the eNB transmits asignal w_(0,0)*S_(k,12) at subcarrier k in OFDM symbol 12 and a spreadversion of the signal w_(0,0)*S_(k,12), w_(0,1)*S_(k,12) at subcarrier kin OFDM symbol 13. To facilitate UE A to detect the data signal, thedirect link DM RS spreading code for OFDM symbol 12 and OFDM symbol 13is subjected to appropriate phase rotation such that the symbol phase ofthe spreading code is 0 in OFDM symbol 12 (for example, a CDM code [1w_(0,1)/w_(0,0)] is used).

Therefore, UE A simply discards only DM RS REs in the last OFDM symbolof a subframe and demodulates/decodes a PDSCH, considering that a datasignal is also carried in DM RS REs of the second last OFDM symbol ofthe subframe. In addition, UE A may demodulate/decode the PDSCH afterdespreading signals carried in DM RS REs of the second slot. UE A usesthe DM RSs of the first slot for PDSCH demodulation of the first/secondslot.

In another method (not shown), the eNB may transmit the data signalSk,12 at subcarrier k in OFDM symbol 12 and a different data signalSk,13 at subcarrier k in OFDM symbol 13. That is, each DM RS RE of thesecond slot may be used for PDSCH transmission without any restriction.In this case, UE A may perform PDSCH demodulation/decoding, consideringthat data signals are transmitted in all DM RS REs of the second slot.

Meanwhile, UE B assumes that a signal for UE A MU-MIMO-operated with asignal for UE B and transmitted in DM RS REs was spread with a codeorthogonal to a DM RS of UE B irrespective of whether the signal for UEA is a data signal or an RS. Therefore, UE B performs channel estimationafter despreading signals in DM RS REs of the second slot.

The above-described operation is also applicable to a subframe with 12OFDM symbols in the case of an extended CP. While the above descriptionhas been given in the context of DM RS Tx enable/disable in the secondslot of a subframe, the same thing applies to the first slot of thesubframe, if DM RS Tx enable/disable is set for the first slot of thesubframe. It is also possible to change a slot to which DM RS Txdisable/enable is applied according to a preset pattern or throughsignaling. For example, a slot for which DM RS Tx is disabled may beindicated by higher layer signaling (e.g. RRC signaling) or via a PDCCHfor DL allocation (e.g. a PDCCH for PDSCH scheduling).

FIG. 19 is a block diagram of a Base Station (BS), an RN, and a UE whichare applicable to the present invention.

Referring to FIG. 19, a wireless communication system includes a BS 110,an RN 120, and a UE 130.

The BS 110 includes a processor 112, a memory 114, and an RF unit 116.The processor 112 may be configured so as to implement the proceduresand/or methods of the present invention. The memory 114 is connected tothe processor 112 and stores various pieces of information related tooperations of the processor 112. The RF unit 116 is connected to theprocessor 112 and transmits and/or receives RF signals. The relay 120includes a processor 122, a memory 124, and an RF unit 126. Theprocessor 122 may be configured so as to implement the procedures and/ormethods of the present invention. The memory 124 is connected to theprocessor 122 and stores various pieces of information related tooperations of the processor 122. The RF unit 126 is connected to theprocessor 122 and transmits and/or receives RF signals. The UE 130includes a processor 132, a memory 134, and an RF unit 136. Theprocessor 132 may be configured so as to implement the procedures and/ormethods of the present invention. The memory 134 is connected to theprocessor 132 and stores various pieces of information related tooperations of the processor 132. The RF unit 136 is connected to theprocessor 132 and transmits and/or receives RF signals. The BS 110, therelay 120 and/or the UE 130 may have a single or multiple antennas.

The embodiments of the present invention described hereinbelow arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

In the embodiments of the present invention, a description is made,centering on a data transmission and reception relationship among a BS,a relay, and an MS. In some cases, a specific operation described asperformed by the BS may be performed by an upper node of the BS. Namely,it is apparent that, in a network comprised of a plurality of networknodes including a BS, various operations performed for communicationwith an MS may be performed by the BS, or network nodes other than theBS. The term ‘eNB’ may be replaced with the term ‘fixed station’, ‘NodeB’, ‘Base Station (BS)’, ‘access point’, etc. The term ‘UE’ may bereplaced with the term ‘Mobile Station (MS)’, ‘Mobile Subscriber Station(MSS)’, ‘mobile terminal’, etc.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the methods according to theembodiments of the present invention may be achieved by one or moreApplication Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, the embodiments of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. For example, software code may be stored in a memory unitand executed by a processor. The memory unit is located at the interioror exterior of the processor and may transmit and receive data to andfrom the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

The present invention relates to a wireless communication system.Particularly, the present invention is applicable to a method andapparatus for transmitting a signal via a relay backhaul link in awireless communication system.

What is claimed is:
 1. A method for transmitting a signal by a basestation (BS) in a wireless communication system, the method comprising:mapping, by the BS, a reference signal to a subframe comprising a firstslot and a second slot, wherein each of the first and second slotsincludes a plurality of resource elements over which the referencesignal is allowed to be mapped, and wherein, when a last orthogonalfrequency division multiplexing (OFDM) symbol of the subframe is notavailable for a BS-to-relay transmission, the reference signal is notmapped to the second slot and a data signal is mapped to the resourceelements of a penultimate OFDM symbol of the second slot; andtransmitting, by the BS, the reference signal to a relay.
 2. The methodaccording to claim 1, wherein, when the last OFDM symbol of the subframeis available for the BS-to-relay transmission, the reference signal ismapped to both of the first and second slots of the subframe.
 3. Themethod according to claim 1, wherein the plurality of resource elementsare resource elements consecutive in time in each of the first andsecond slots, and the plurality of resource elements are located in lasttwo OFDM symbols of a corresponding slot.
 4. The method according toclaim 1, wherein the reference signal is spread with an orthogonal code.5. A base station (BS) in a wireless communication system, the BScomprising: a radio frequency (RF) unit; and a processor operablyconnected to the RF unit and configured to: map a reference signal to asubframe comprising a first slot and a second slot, wherein each of thefirst and second slots includes a plurality of resource elements overwhich the reference signal is allowed to be mapped, and wherein, when alast orthogonal frequency division multiplexing (OFDM) symbol of thesubframe is not available for a BS-to-relay transmission, the referencesignal is not mapped to the second slot and a data signal is mapped tothe resource elements of a penultimate OFDM symbol of the second slot,and transmit the reference signal to a relay.
 6. The BS according toclaim 5, wherein, when the last OFDM symbol of the subframe is availablefor the BS-to-relay transmission, the reference signal is mapped to bothof the first and second slots of the subframe.
 7. The BS according toclaim 5, wherein the plurality of resource elements are resourceelements consecutive in time in each of the first and second slots, andthe plurality of resource elements are located in last two OFDM symbolsof a corresponding slot.
 8. The BS according to claim 5, wherein thereference signal is spread with an orthogonal code.
 9. A method forreceiving a signal by a relay in a wireless communication system, themethod comprising: receiving, by the relay from a base station (BS), areference signal in a subframe comprising a first slot and a secondslot, wherein each of the first and second slots includes a plurality ofresource elements over which the reference signal is allowed to bemapped, and wherein, when a last orthogonal frequency divisionmultiplexing (OFDM) symbol of the subframe is not available for aBS-to-relay transmission, the reference signal is not received in thesecond slot and a data signal is received in the resource elements of apenultimate OFDM symbol of the second slot.
 10. The method according toclaim 9, wherein, when the last OFDM symbol of the subframe is availablefor the BS-to-relay transmission, the reference signal is received inboth of the first and second slots of the subframe.
 11. The methodaccording to claim 9, wherein the plurality of resource elements areresource elements consecutive in time in each of the first and secondslots, and the plurality of resource elements are located in last twoOFDM symbols of a corresponding slot.
 12. The method according to claim9, wherein the reference signal is spread with an orthogonal code. 13.The method according to claim 9, further comprising: decoding, by therelay, the data signal based on the reference signal.
 14. A relay in awireless communication system, the relay comprising: a radio frequency(RF) unit; and a processor operably connected to the RF unit andconfigured to receive a reference signal from a base station (BS) in asubframe having a first slot and a second slot, wherein each of thefirst and second slots includes a plurality of resource elements overwhich the reference signal is allowed to be mapped, and wherein, when alast orthogonal frequency division multiplexing (OFDM) symbol of thesubframe is not available for a BS-to-relay transmission, the referencesignal is not received in the second slot and a data signal is receivedin the resource elements of a penultimate OFDM symbol of the secondslot.
 15. The relay according to claim 14, wherein, when the last OFDMsymbol of the subframe is available for the BS-to-relay transmission,the reference signal is received in both of the first and second slotsof the subframe.
 16. The relay according to claim 14, wherein theplurality of resource elements are resource elements consecutive in timein each of the first and second slots, and the plurality of resourceelements are located in last two OFDM symbols of a corresponding slot.17. The relay according to claim 14, wherein the reference signal isspread with an orthogonal code.
 18. The relay according to claim 14,wherein the processor is further configured to decode the data signalbased on the refrence signal.